Circular Polarization Cross Slot

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IEEE

TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL 44, NO. 10, OCTOBER 1996

1399

A

Circularly Polarized Small-Size

Microstrip Antenna with a Cross Slot

Hisao Iwasaki ,

Member

IEEE

Abstruct- A new, circularly polarized small-size microstrip

antenna using a proximity coupled feed method is proposed.

A simple configuration based on

a

cross slot with unequal slot

lengths on a circular patch is adopted to realize a small-size

element antenna. The proposed antenna has no 90 hybrid

coupler for circular polarization. The measured results verify

the circular polarization, and the antenna radius was reduced

by about 36 by using the slot lengths which are nearly equal

to the diameter of the circular patch antenna. Good impedance

and axial ratio characteristics have been obtained.

I.

INTRODUCTION

circularly polarized antenna with a low profile, small

A

ize, and light weight is required in mobile satellite

comm unications. Many types of m icrostrip antennas have been

proposed and ivnestigated

[

11.

Circularly polarized microstrip antennas are classified as

single-fed type or dual-fed type, depending on the n umber

of feed points necessary to generate the circularly polarized

waves. Th single-fed type has the advantage of not requiring an

external polarizer such as a 90 hybrid coup ler. The relation-

ship between the optim um probe location and the frequen cy of

the ob tained circularly polarized wave has b een clarified, and

good exp erimental results have been reported

[ 2 ] .

Recently,

aperture-coupled feed m ethods have been attracting much

attention because their geo metries are suitable for mono lithic

integration with microwave or milliwave devices. The feed

position for a circularlly polarized operation and the input

impedance of several microstrip antennas fed by the slot-

coupled method have been investigated [3]. However, with

this type of microstrip antenna it is difficult to excite good

circularly polarized w aves.

Another suitable feed method is

an

electromagnetically

coupled method which is also known as the proxim ity-coupled

method. This type of antenna has several advantages over

a directly fed patch antenna. By using a proximity-coupled

method, an optimal feedl point of a microstrip antenna has been

proposed for linear polarization

[4].

Moreover, a circularly

polarized rectangular microstrip antenna, fed by proximity

coupled method using an offset microstrip line, was propo sed

by the author

[5] .

On the other hand, the element of a phased-array antenna

must be arranged at almost about half wavelength to obtain

wide-angle beam scanning. The resonant frequency of a single-

fed circularly polarized microstrip antenna with a thin diagon al

Manuscript received September 6, 1995.

The author is with Toshiba Research and Development Center, Communica-

tion and Information Systems Research Laboratones, 1 Kom ukai Toshiba-Cho,

Saiwai-Ku, Kawasalu 210, Japan.

Publisher Item Identifier S 0018-926X(96)07552-7.

-f

Fig. 1. Configuration

of

the proposed patch antenna.

center slot on the patch radiator was almo st the same as that of

a patch antenna with a 90 hybrid [6]. The refor e, it is difficult

to arrange an antenna element at about half wavelength in the

case of the above-patch antenna.

The purpose of this paper is to propose a small-size circular

patch antenna using a cross slot with unequ al slot lengths. The

proposed antenna can a chiefe circular poarization without the

need for a 90 hybrid cou pler. The m easured results are pre-

sented to dem onstrate lth usefulness

of

the proposed antenna

configuration. Good impedan ce and axial ratio characteristics

are realized.

11. ANTENNA ONFIGURATION

The propo sed antenna configuration s shown in Fig. 1. The

circular patch with a cross slot and the microstrip line are

formed by the su bstrates with a dielectric constant

t~

and

thickness

h l

and h2 respectively. The radius of the circular

patch

is

T Slot A with length Lsa and slot

B

with length

Lsb cross orthogonally at the center of each slot, which is the

center of the circular patch. The characteristic imp edance of

the m icrostrip line is

50

R. 5 1 is the distance between the end

of the m icrostrip line and the center of the patch antenna.

111.

EXPERIMENTAL

ESULTS

The resonant frequency of the linearly polarized circular

patch with a slot can be controlled by changing the slot

0018-926X/96$05.00 996

IEEE



1400

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 44, NO. 10, OCTOBER 1996

f 1.525GHz

r

=33.31

w 4.5

mm

SI =25 0 mm

h l

1.6 mm

h 2

1.6 mm

n

U

Frequency

[

GHz

Fig.

2.

Measured input impedance and return

loss.

:

.=33.31mm

2dB

h2

=1.6

l s

mm

I

I I

I

1.5

1.51

1.521.53

1.541.55

'

Frequency

K H z l

Fig. 3.

Measured axial ratio as a parameter

of

the slot length Lsb.

length

[7].

The resonant frequency decreases monotonically

with increasing slot length. Therefore, resonant frequencies of

orthogonal modes, as a result of the perturbation caused by a

cross slot, as shown in Fig.

1,

will decrease with increasing

slot lengths

Lsa

and

Lsb.

Thus, the resonant frequency of

the proposed patch antenna can be reduced. Consequently,

the proposed antenna can be made small size and compact

compared with a linearly polarized patch antenna with a single

slot.

The proposed antenna was designed and tested to verify

circularly polarizing operation. The experimental m odels were

made of copper-clad substrate with ET

=

2.6 and the thickness

h l

=

h2

=

1.6 mm. The width of the microstrip line W

was 4.5 mm. The radius of the circular patch without a cross

slot was

T = 33.31

mm, and the resonant frequency was 1.55

GHz The patch antenna was designed by using the simple

cavity method [8].

X

-30

90 -45 45

90

8 [ d e g l

Fig. 4. Measured radiation pattem z plane,

= 1 . 525

GHz)

Fig. 2 shows the measured impedance and return loss for

the

Lsa

= 20.0 mm,

Lsb

= 12.0 mm, and S = 25.0

mm. Reference plane is the edge of the circular patch. Good

impedance matching was obtained. The bandwidth VSWR less

than 2 was 3.4%. The resonant frequency of the circular patch

antenna without a cross

slot was 1.55 GHz. Therefore, the

antenna radius can be reduced by about 2% using the cross-

slot configuration compared with that of the circular patch

without a slot.

Fig. 3 shows the measured axial ratios as a parameter of the

slot length

Lsb.

A 0.5-dB boresight axial ratio was obtained

at 1.525 GHz when

Lsa = 20 0

mm and

Lsb

= 12.0 mm.

Th e 2-dB bandwidth of axial ratio w as 0.65%. This bandwidth

was almost the same as that of the conventional patch antenna.

The measured gain was 6.0 dBi at the boresight.

Fig. 4 shows the radiation pattern in the z plane at 1.525

GHz. An axial ratio of about 3.0 dB was obtained in the f.60

range.

To verify the possibility of achieving a reduction of the

antenna size by using this proposed method, an antenna with

Lsa = 61.0 mm and Lsb = 60.5 mm, nearly equal to the

diameter of the circular patch antenna, was tested. Fig. 5 show s

the radiation pattem in the x plane at

0.98 GHz.

0.5-

dB boresight axial ratio was obtained at 0.98 GHz. An axial

ratio of about 3.0 dB was obtained in the &45 range. Thus,

the radius of the circular patch with a cross slot was reduced

by about

36%

compared with the radius of a circular patch

without a cross slot. The gian was about 7.5 dB lower than

that the theretical value of a probe-fed patch antenna w ithout

a cross slot.

IV. CONCLUSION

This paper describes the results of measurements of a new

single-fed circularly poalrized microstrip antenna. A small-size

element antenna for circular polarization was realized by using

a circular patch antenna with a cross slot having different arm

lengths. The antenna radius was reduced by about 36% by



IWASAKI: CIRCULARLY POLARIZED SMALL-SIZE MICROSTRIP ANTENNA WITH A

CROSS

SLOT

1401

n

m

‘ b o

-10

d) -20

-30

>

.

B

’

= 3.98GHz

’ 33.31”

Ws=

2.0mi

Y 4.5mm SI =25.0mi

sa=61O

mm

h 1.6mi

-sb=6Q5 mm

h 2 =

1.6~11

I

3 -45 45 90

tdegl

Fig. 5. Measured radiation pattern z plane, f = 0.98 GHz .

using slot lengths which are nearly equal to the diameter of

the circular patch antenna.

The proposed antenna is suitable for application in the field

of mob ile satellite comm unications as a phased-array antenna

using a multilayered feed network integrated with microw ave

devices.

REFERENCES

[l ] F. Naderi, “NASA’S mobile satellite communications program: Ground

and space segment technologies,” Los Angeles, CA, IAF-84-84, 1984.

[ ] Y. Suzuki,

N. Miyano, and T. Chiba, “Circularly polarized radiation

from singly fed equilateral-triangular microstrip antenna,” IEE Proc.

vol. 134, pt. H, pp. 194-198, Apr. 1987.

[3] W.

H.

Aksun, Z .

H.

Wang, S. L. Chung, and

Y .

T. Lo, “On slot-coupled

microstrip antennas and their applications CP operation theory and

experiment,” IEEE Trans. Antennas Propagat. vol. 38, pp. 1224-1230,

Aug. 1990.

[4] M. Davidovitz and Y. T. L o, “Rigorous analysis of

a

circular patch an-

tenna excited by a microstrip transmission line,” IEEE Trans. Antennas

Propagat.

vol. 37, pp. 949-958, A ug 1989.

[5] H. Iwasaki,

H.

Sawada, and K. Kawabata, “A circular polarized

mi

crostrip antenna using singly-fed proximity coupled feed,” in Proc. Int.

Symp. Antennas P ropagat. Sapporo, Japan, Sept. 1992, pp. 797-800.

[6] J. Ken, “Microstrip antenna developments,” W orkshop Antenna Tech-

nol., New Mexico State Univ. Las Cruces, pp. 3.1-3.20, 1979.

[7] H. Iwasaki and K. Kawabata, “A circularly polarized microstrip antenna

with a cross slot.” in 3rd Antennas Prooaaat. Meet. Con f Proc. Tokyo,

Japan, Sept. 1990, pp. 281-284.

181

I. J. Bahl

and

P.

Bhartia, Microstrip Antenna. Nonvood, M A: Artech

House, 1980.

Hisao Iwasaki (M’86) received the

B.S.

and D.E.

degrees in electrical enginering from Tohoku Uni-

versity, Sendai, Japan, in

1975

and 1994, respec-

tively.

In 1975, he joined the Toshiba Research and

Development Center, Kawasalu, Japan, where he

did research

on

antennas for communications.

In

September 1986, he joined ATR Optical and Ra-

dio Comm unications Research Laboratories, K yoto,

Japan, where he was engaged in research on active-

array antennas.

He

is currently a Senior Research

Scientist at C ommunication and Information Systems Research Labs, Toshiba

Research and Development C enter, Kawasaki-City, Japan, where he has been

engaged in the development of the microstrip antennas for comm unications.

Dr. Iwasaki is a member of IEICE.

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Circular Polarization Cross Slot